Treatment of aluminum oxide surfaces with molten vanadium pentoxide

ABSTRACT

Surface irregularities of a crystalline aluminum oxide member, such as an Alpha -corundum crystal, are removed by treatment with molten vanadium pentoxide, preferably at temperatures exceeding 800* C. The treatment may be carried out in more than one stage, using a fresh vanadium pentoxide melt in each stage.

United States Patent Inventors Marc Marian Faktor Bushey; GeorgeReginald Newns, Ealing, London; Dennis Gordon Fiddyment, London, all ofEngland Appl. No. 700,898

Filed Jan. 26, 1968 Patented Nov. 9, 1971 Assignee Her MajestysPostmaster General London, England Priority Feb. 3, 1967 Great Britain5,347/67 TREATMENT OF ALUMINUM OXIDE SURFACES WITH MOLTEN VANADIUMPENTOXIDE 12 Claims, 6 Drawing Figs.

U.S. Cl 156/2, 156/6, B/Sl, 23/52 Int. Cl C231 1/04, C23g 1/00 Field ofSearch 23/141, 51,

[ 56] References Cited UNlTED STATES PATENTS 2,510,219 6/1950 Graham156/2 OTHER REFERENCES P. H. Robinson et al., The Deposition of Siliconupon Saphire Substrates Trans. of the Metallurgical Soc. of AIME, Vol.236, Mar., 1966, pp. 268- 274 cited.

Fink, Reaction of V205 with aluminia Berichte, Nov., 1964, pp. 627- 631.

Rostoker, The Metallurgy of Vanadium, pps. 52, 53 and 60 cited,published 1958.

Anderson, Studies on Vanadium Oxides," Acta Chemica Scandnavica, Aug.,1954; No. 9, pp. 1,599- 1,606 cited.

Primary Examiner Robert F. Burnett Assistant Examiner-R. .1. RocheAttorney-Hall and Houghton ABSTRACT: Surface irregularities of acrystalline aluminum oxide member, such as an a-corundum crystal, areremoved by treatment with molten vanadium pentoxide, preferably attemperatures exceeding 800C. The treatment may be carried out in morethan one stage, using a fresh vanadium pentoxide melt in each stage.

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ATTORNEY TREATMENT OF ALUMINUM OXIDE SURFACES WITI-I MOLTEN VANADIUMPENTOXIDE This invention relates to improving the surface properties ofcrystalline forms of aluminum oxide.

The present invention provides a process for the treatment ofcrystalline aluminum oxide which comprises contacting at least onesurface of a crystalline aluminum oxide member with molten vanadiumpentoxide in such manner that a portion of the surface is removed.

More especially, the present invention provides a process for theproduction of a clean polished surface on a crystalline form of aluminumoxide, especially a-corundum, which comprises polishing said surface,more especially lapping and mechanical polishing by abrasives, andsubsequently contacting said surface with molten vanadium pentoxide soas to remove the surface to a sufficient depth that underlying scratchesand regions of strain and disorder are substantially removed.

Advantageously the process of the invention is carried out with themolten vanadium pentoxide at a temperature in the range of from 800 tol,000 C.

In the preferred form of the invention in which the process is appliedto the production of a clean polished surface, the surface of thealready polished aluminum oxide is preferably immersed in moltenvanadium pentoxide for a period of from minutes to 2 hours and is keptstationary without agitation of the melt. The specimen may be rotatedbut it has been found that, although rotation causes more rapid removalof a given depth of surface, the resulting surface is less smooth thanthe surface produced when the crystalline specimen is kept stationary.

Optical assessment and microprofilometer (Talysurf) measurements showthat the initial surface after mechanical polishing, although smooth,possesses numerous scratches. As dissolution takes place the scratcheswiden and become more marked and then disappear, leaving a substantiallyfeatureless surface after a sufficient depth of surface has beenremoved. In experiments with a-corundum it was found that removal ofabout 30 am. of the surface of the aluminum oxide was necessary beforethe rate of removal became constant, indicating that the underlyingscratches and regions of disorder had been removed. As further surfaceis removed the resulting surfaces become somewhat striated inappearance, although the microprofilometer shows no roughening.

The rate of removal of surface increases with the melt temperature, buta higher melt temperature also increases the degree of striation on theresulting surfaces. It was found that for a-corundum the removal ofadepth of 30 um. took about 1 hour at 800 C., about 30 minutes at 900 C.and about l5 minutes at 1,000 C.

Increased concentration of alumina in the melt reduces the rate ofdissolution considerably and for this reason it is advantageous, when itis desired to remove substantial depths of surface, to carry out theprocess of the invention in a plurality of stages in each of which afresh vanadium pentoxide melt is employed.

It has been found that the removal of a given depth of surface, nomatter how it is effected so far as temperature, time of immersion andalumina concentration in the melt are concerned, produces a similardegree of smoothness as assessed by a microprofilometer.

in the melt. This film is in general resistant to chemical attackalthough there are slight changes in its appearance after treatment withconcentrated hydrochloric nitric or hydrofluoric acids, or a solution ofcatechol in hydrazine hydrate. The films disappear after prolongedheating in air at l,200 C.

In some industrial applications, it is important that crystalline formsof aluminum oxide should have a clean polished surface free from workdamage. For example, it is advantageous for a-corundum to have such asurface for the epitaxial deposition of thin films of silicon. Highlypolished surfaces produced by lapping and polishing with abrasives,however fine, have underlying scratches and associated regions of strainand disorder beneath the surface which may adversely influence thedeposit.

For the production of a polished clean surface on a crystalline specimenof aluminum oxide, the specimen is advantageously lapped andmechanically polished with a final abrasive of diamond dust having aparticle size in the range of from 0.2 pm. to 0.8 pm, for example, about0.5 am, in the process of the invention. The specimen is then suspendedfrom a fine platinum wire and immersed slowly in a crucible of moltenvanadium pentoxide at a temperature in the range of from 800 to l,000 C.It is kept stationary for from 15 minutes to 2 hours, removed andallowed to cool slowly to avoid thermal shock. The residual vanadiumpentoxide is then removed from the specimen by immersion in hotconcentrated hydrochloric acid and the specimen is washed in distilledwater and alcohol and finally dried in air.

It is to be understood that references to crystalline forms of aluminumoxide throughout this specification include those forms, for example,ruby and sapphire, which contain traces of other materials, for example,metal oxides.

The following example illustrates the invention:

EXAMPLE A l cm. wafer of a-corundum was cut about 1 mm. thick from alarge Verneuil-grown single crystal, perpendicular to the c-axis. Eachface was lapped and polished with a final abrasive of 0.5 pm. diamonddust. The specimen was then cleaned by washing with distilled waterfollowed by ethyl alcohol, dried in hot air at about 60 C. and weighed.

The specimen was then immersed and kept stationary in an unstirred meltof vanadium pentoxide at 800 C. After 2 minutes, it was removed from thevanadium pentoxide melt, allowed to cool, cleaned of solidified vanadiumpentoxide by immersion in hot concentrated hydrochloric acid, washed indistilled water and alcohol, dried in air and finally weighed. It wasfound from the loss in weight that a depth of surface equal to 1.5 gm.had been removed.

The results of a series of similar experiments carried out on otheridentically prepared crystalline specimens are shown in table 1. Theresults show that, other conditions being equal, the greater thetemperature the greater is the depth of surface removed, and also thatthe rate of removal of surface is retarded by an increased concentrationofalumina in the melt.

In FIG. 1 of the accompanying drawings traces (a) to (f) aremicroprofilometer (Talysurf) assessments of the original surface (a) andthe surface of some of the crystalline specimens after the removal, inthe above series of experiments, of a depth of surface of 1.5 pm. (b),3.9 pm. (c), l4.7 um. (d), 33.4 am. (e) and 126 um. (f).

TABLE 1 Temperature of Total time of Depth of vanadium pentoximmersionof surface re ide melt, crystal moved, pm Remarks 800.. 2 mins. l. 5800.. 2 hours 3.0 Melt contained dissolved alumina.

2 mins. 3. El 900... l0mins 10 900.. 12mins... 14.7 900.. 32 mins...33.4

800. 5hr.2mins 52.0 One-stage immersion. Alumina in melt allowed toaccumulate. 000.. 1hr. 30 mins... 108 Three iminersions of 30 minuteseach, using a fresh vanadium pentoxide melt for each innnersion. 000.. 3hr. 2 mins... 126 Four imlnersions of 2 minutes, 10 minutes, 20 minutesand minutes duration, using a fresh vanadium pentoxide melt for eachimmersion.

We claim:

A pale violet film appears on some surfaces after immersion 1. A processfor the treatment of crystalline aluminum oxide in which at least onesurface of a crystalline aluminum oxide member is contacted with avanadium pentoxide melt having a temperature in the range of from 800 C.to 1,000 C., the contacting being continued at least for a sufficienttime to remove the surface to a sufi'icient depth that underlyingscratches and regions of stress and disorder are substantially removed.

2. The process according to claim 1 in which the surface is lapped andmechanically polished prior to being contacted with the vanadiumpentoxide melt.

3. The process according to claim 1 in which the member is keptsubstantially stationary whilst the surface is in contact with thevanadium pentoxide melt.

4. The process according to claim 1 in which after being contacted withthe vanadium pentoxide melt the member is washed in hot concentratedhydrochloric acid, is washed in distilled water, is washed in alcoholand then is dried in air.

5. The process according to claim 1 in which the surface is given afinal mechanical polish with diamond dust having a particle size in therange of from 0.2 pm. to 0.8 pm.

6. The process according to claim 1 in which the contacting of thesurface of the member with a vanadium pentoxide melt is carried out in aplurality of stages, in each of which a fresh vanadium pentoxide melt isused.

7. A process for the treatment of crystalline aluminum oxide in which atleast one surface of a crystalline aluminum oxide member is contactedwith a vanadium pentoxide melt having a temperature in the range of from800 C. to 1,000 C. until a depth of surface of at least 30 pm. has beenremoved.

8. The process according to claim 7 in which the surface is lapped andmechanically polished prior to being contacted with the vanadiumpentoxide melt.

9. The process according to claim 8 in which the surface is given afinal mechanical polish with diamond dust having a particle size in therange of from 0.2 pm. to 0.8 pm.

10. The process according to claim 7 in which the member is keptsubstantially stationary whilst the surface is in contact with thevanadium pentoxide melt.

11. The process according to claim 7 in which after being contacted withthe vanadium pentoxide melt the member is washed in hot concentratedhydrochloric acid, is washed in distilled water, is washed in alcoholand then is dried in air.

12. The process according to claim 7 in which the contacting of thesurface of the member with a vanadium pentoxide melt is carried out in aplurality of stages, in each of which a fresh vanadium pentoxide melt isused.

2. The process according to claim 1 in which the surface is lapped andmechanically polished prior to being contacted with the vanadiumpentoxide melt.
 3. The process according to claim 1 in which the memberis kept substantially stationary whilst the surface is in contact withthe vanadium pentoxide melt.
 4. The process according to claim 1 inwhich after being contacted with the vanadium pentoxide melt the memberis washed in hot concentrated hydrochloric acid, is washed in distilledwater, is washed in alcohol and then is dried in air.
 5. The processaccording to claim 1 in which the surface is given a final mechanicalpolish with diamond dust having a particle size in the range of from 0.2Mu m. to 0.8 Mu m.
 6. The process according to claim 1 in which thecontacting of the surface of the member with a vanadium pentoxide meltis carried out in a plurality of stages, in each of which a freshvanadium pentoxide melt is used.
 7. A process for the treatment ofcrystalline aluminium oxide in which at least one surface of acrystalline aluminium oxide member is contacted with a vanadiumpentoxide melt having a temperature in the range of from 800* C. to1,000* C. until a depth of surface of at least 30 Mu m. has beenremoved.
 8. The process according to claim 7 in which the surface islapped and mechanically polished prior to being contacted with thevanadium pentoxide melt.
 9. The process according to claim 8 in whichthe surface is given a final mechanical polish with diamond dust havinga particle size in the range of from 0.2 Mu m. to 0.8 Mu m.
 10. Theprocess according to claim 7 in which the member is kept substantiallystationary whilst the surface is in contact with the vanadium pentoxidemelt.
 11. The process according to claim 7 in which after beingcontacted with the vanadium pentoxide melt the member is washed in hotconcentrated hydrochloric acid, is washed in distilled water, is washedin alcohol and then is dried in air.
 12. The process according to claim7 in which the contacting of the surface of the member with a vanadiumpentoxide melt is carried out in a plurality of stages, in each of whicha fresh vanadium pentoxide melt is used.